1. Field of the Invention
The present invention relates to a sheet processing machine including a sheet braking device and a method for the braking of sheets to a standstill.
2. Description of the Related Art
Sheet processing machines usually possess an input device, processing stations, and a delivery device. From a stack of sheets located in the input device, a sheet lying face up is singled out and passed off to a transport system. The transport system transports the sheet through the processing stations and to the delivery device. A known transport system has circulating gripper carts, each of which consists of a cross rod on which grippers are mounted which are used to grab the sheet by the front edge and whose ends are secured on a chain drive at the sides which carries the gripper carts through the machine. For the processing and the delivery steps, the sheet must be braked to a standstill. This is done, first, by braking the gripper cart and also, in addition, by additional sheet brakes. The sheet processing machines can be, for example, sheet printing machines or, in particular, sheet punching and embossing machines.
Punching refers to a cutting with closed, geometrical forms, which can be circular, oval, or polygonal, as well as special shapes of every kind. Practices in the post-processing of a print job, such as punching with a hollow punch, rounding corners, and register punching are also included in this field. The punching is done against a base or against a punch, and sometimes there are also shearing processes (cf. Post-processing, Training Manual for Bookbinders, Bundesverband Druck e.V. 1996, page 351 et seq.) involved. Packaging materials of paper, cardboard, paperboard or corrugated cardboard are primarily punched in sheet format. During the punching process, however, groove lines or blind imprints may also be introduced into the finished sheets. This complex process makes it essential to punch the sheets individually. Since the end products are packages with demanding technical and graphical requirements (such as those for cosmetics, cigarettes, medicines, foods, etc.), special requirements are placed not only on the packaging materials themselves, but also one needs punching dies with minimal tolerances and extremely precise and reliable punching machines for optimal results. These demands are best met by flat bed punches. The printed sheets, stacked on a pallet, are fed to the punch. In the machine, the sheets being punched are first oriented accurately in an orienting mechanism, picked up by a gripping cart and positioned exactly in the punching mechanism between a firmly mounted bottom table and a top table, able to move vertically via a bent lever or eccentric gearing.
Such a flat bed punch is known, for example, from DE 30 44 083 A1. The two tables are provided with cutting and grooving dies and corresponding counterplates, with which the finished copies are punched out from the sheets carried between the table surfaces in a timed cycle, and at the same time, the grooves needed for making a clean fold are indented. In the subsequent breakaway mechanism, the waste is mechanically removed via breakaway dies. Finally, depending on the machine construction, the punched copies can be separated in a copy separation mechanism.
Both in the breakaway station and in the copy separation station with sheet delivery, the sheet must be braked from the transport speed down to a standstill. Since the sheet, weakened by the preceding cut, enters the station at high speed, mere slowing down of the front gripper rod can cause a pushing up of its rear part, or so-called “bulging” of the sheet. This should be prevented by additional braking devices, which act on the surface of the sheet. The taut sheet can then be processed with greater precision or laid aside with edge aligned.
Various types of sheet braking devices are known in the prior art.
EP 1 108 671 A2 shows a sheet braking system for a delivery unit of a sheet processing machine with a circulating brake band. The brake band has a suction region, which exerts a suction action on a sheet being braked. The brake band periodically runs through deceleration and acceleration phases, i.e., it is moved in time with the sheet transport. During the course of a deceleration phase of a sheet, the braking section of the band is covered by the sheet being braked.
DE 42 18 421 A1 shows the sheet guide in the delivery unit of a sheet printing machine. Two gripping systems are used here and include a front-edge gripping system and a back-edge gripping system, each of which has sheet grippers for grasping the edges of the sheet. The simultaneous guiding of a sheet at its front and its rear edge makes it possible to transport and brake the sheet safely and free of damage.
In an alternative solution, pneumatic sheet brakes are used, as is known from DE 102 59 556 A1. These sheet brakes are situated in direct proximity to the sheet and provide a partial vacuum, which acts as a braking force on the sheet moving past the pneumatic sheet brake. Pneumatic sheet brakes have the advantage over mechanical sheet brakes that the surface of the sheet is not impaired, and thus one can avoid leaving marks on the sheet. The mode of operation of pneumatic sheet brakes is based on the Venturi effect.
DE 695 00 514 T2 shows, for example, a sheet braking device with brushes. The braking brushes are slanted relative to the direction of sheet transport and exert by light pressure a braking force on the sheet. The drawback is that this may leave marks on the sheet.
DE 10 2004 022 235 A1 discloses a device for delivery of sheets in a sheet delivery unit of a sheet printing machine. The device is based on both a sheet brake using friction to act on the entire width of the sheet, and a removal gripper and suction bar, which are located in the rear area of the delivery unit. Since the sheets, for an optimal placement, are to be grabbed and then released at different moments of time, depending on the operating mode or material properties, the suction bar is arranged such that it can move in the sheet transport direction. The problem with this design of a sheet brake is that the suction bar, being movable, is subject to more intense wear and tear. Furthermore, such a sheet brake cannot be used in the processing stations of a sheet processing machine, or else there will be a collision between processing dies and suction bar. But it is necessary to adapt the sheet brake to the sheet format in order to make sure that the sheet brake acts on the sheet until shortly before it reaches standstill.
What is common to all sheet braking systems known in the prior art is that they are located in the entry zone of the processing stations and delivery unit and thus are arranged directly in front of the delivery surface or processing dies. In practice, however, the lengths of delivery unit and processing stations do not always correspond to the sheet lengths being currently used. Thus, subformat sheets cannot be braked to a standstill by the sheet braking system, since the sheets are transported by the front edge to a defined end position and therefore are no longer situated in the active zone of the sheet braking system. In order to brake the sheets nonetheless to a standstill, they generally need to be braked strongly by the front edge.